Designing Liquid Silicone Rubber Prototypes and Components
We offer liquid silicone rubber (LSR) among our list of injection molding capabilities. LSR molding shares many similarities with conventional injection molding, but there are a few notable differences. Unlike thermoplastic resin, which is melted before injection, LSR is a two-part thermoset compound that is chilled, before being injected into a heated mold and ultimately cured into a final part. Since LSR is a thermosetting polymer, its molded state is permanent—once it is set, it can’t be melted again like a thermoplastic.
LSR has certain inherent characteristics. It is a strong, elastic material with excellent thermal, chemical, and electrical resistance. LSR parts also maintain their physical properties at extreme temperatures and can withstand sterilization. LSR is biocompatible so it works very well for products that have skin contact. Those benefits lend themselves well to automotive, medical and food appliance industries, typically in the form of seals, gaskets, valves, and cables.
Designing parts for LSR and thermoplastics are similar, but there are some LSR-specific guidelines to consider:
We allow for a maximum LSR part size of 12 in. (304mm) by 8 in. (203mm) by 4 in. (100mm) with depths no greater than 2 in. (50mm) from any parting line. Note that deeper parts are limited to a smaller outline. Maximum surface area is 48.4 sq. in. (312 sq. cm.) and maximum volume is 13.3 cu. in. (217 cc).
Wall and Rib Thickness
LSR typically fills thin wall sections with minimal challenges, and walls as thin as 0.010 in. are possible, depending on the size of the wall and the location of adjacent thicker sections. Rib thickness should be 0.5 to 1.0 times the adjoining wall thickness. LSR is accommodating to variations in wall thickness and sink is almost nonexistent.
Shrink and Flash
The shrink rate on LSR is fairly high with an expected tolerance of 0.025 in./in. LSR also tends to flash very easily during molding (in gaps as small as 0.0002 in.), which Proto Labs helps reduce by incorporating additional features into the mold design.
Simplifying and minimizing parting lines in your design will help you get cleaner LSR parts as quickly as possible.
LSR can be molded to accommodate parts with undercuts, which are manually removed by a press operator. Mechanical tooling actions to release undercuts are selectively offered at Proto Labs.
Ejector pins are normally not used during LSR molding due to the flashy nature of the material. Thus, parts should be designed so they can be retained on one half of the mold when it is opened at the end of the molding cycle. The part is then manually de-molded, often with air assistance.
LSR has been in the industry for a long time, but we offer LSR parts in volumes of 25 to 5,000+ in three weeks or less. To learn more about our LSR capabilities—including additional design guidelines on draft, finishes, and more—check out our LSR page. If you have a 3D CAD model ready, upload it now to get an interactive quote with design analysis and pricing information within hours.